1. Field of the Invention
The present invention relates to an electric discharge machine (wire-cut electric discharge machine) using a wire electrode as tool electrode or an electric discharge machine (electric discharge machine for die sinking) using an electrode formed in a shape corresponding to the shape of an intended workpiece as tool electrode.
2. Description of the Related Art
When positive and negative electrodes approach each other, arc discharge occurs and thus part of the electrodes melts and scatters. Electric discharging utilizes the property. One of the electrodes is an object to be machined (workpiece) and the other is a machining electrode provided in an electric discharge machine.
In a conventional electric discharge machine having an electric discharging power supply which employs a system called “transistor type”, as disclosed in Japanese Patent Application Laid-Open No. 2004-195562 (corresponding to US 2004/0124189 A1), a first power supply having a high output impedance applies a voltage to a machining gap formed by a machining electrode and a workpiece opposite each other, thereby inducing arc discharge. Thereafter, a current pulse is supplied to the machining gap from a second power supply having a low output impedance to generate high-temperature arc, thereby melting and removing the workpiece. After a predetermined OFF time has elapsed, a voltage is applied again by the first power supply. The above operations are repeated thereby to perform machining.
With the electric discharging by the system described above, it is difficult to previously know where discharge occurs in the machining gap or to positively control where the discharge occurs. In general, as electric field intensity is high at a locally-narrow region in the machining gap distance and insulation breakdown easily occurs, a possibility of discharge generation at the part increases. When the discharge occurs and that part is removed, the discharge generating position shifts to a narrower region in the machining gap distance, as a result, the machining gap distance becomes substantially constant as a whole.
However, when a workpiece in the form of a plate is machined by the electric discharging, an electric field intensity becomes high at an edge portion of the workpiece, as shown in FIG. 1, and thus the possibility of discharge generation at the portion increases. A sludge density is also high at the edge, similar to the facing region between the tool electrode and the workpiece, and thus discharge via sludge also easily occurs. Consequently, the discharge occurs not only between the surface of the tool electrode and that part of the surface of the workpiece which directly faces the tool electrode, but also at the edge of the top or bottom surface of the workpiece, as shown in FIG. 2, and thus a failure such as crack easily occurs.
For a press die to be machined, its edge is important for cutting blade, and failures such as a decrease in dimension accuracy and machining defect of the edge directly influence the accuracy or quality of the press machining and thus the failures need to be eliminated as much as possible.
Since a power is supplied to a wire electrode, in wire-cut electric discharging, from two parts including an upper guide and a lower guide, a discharge generating position can be inferred based on a difference in supplied a current between the upper and lower wire guides. Thus, the technique is applied and an attempt is made as exemplified in WO2007/032114 (corresponding to US 2008/0110865 A1) in which a discharge generating position is obtained based on a difference in a current between the upper and lower wire guides by a first power supply and a magnitude of a current pulse to be supplied from a second power supply is adjusted based on the obtained discharge generating position, thereby positively controlling a machining shape. However, in the technique, an accuracy for detecting the discharge generating position is low enough to discriminate the upper, middle and lower parts of a thick workpiece of 50 mm or more in a practical viewpoint, so this technique is somewhat effective in controlling the straightness accuracy of thick punches but not so effective in detecting the discharge generating position at millimeter order accuracy. Thus, with the technique described in the document, the discharge generation at the edge cannot be accurately detected and thus the machining quality of the edge cannot be enhanced.
Generally, in the electric discharging, a clearance of several to several tens micrometers is present in the machining gap during the machining, a voltage of several tens to several hundreds volts is applied to the clearance, the insulation of the clearance is broken by the applied voltage so that pulse current for causing arc discharge is applied, and a high heat due to the arc discharge is utilized to remove the workpiece.
Therefore, even when a machining current pulse is supplied while the machining electrode and its opposite workpiece contact each other and are shorted, the arc discharge does not occur and a current simply flows, which is considered as less contributive to the machining. Thus, when a short-cut has occurred, various attempts has been carried out for the purpose of early elimination of the state of such short-circuit due to stoppage of flow of the pulse current or a concentration of sludge by specially applying a pulse current for scattering the sludge, as described in, for example, Japanese Patent Application Laid-Open No. 7-156019.
As described above, in a conventional electric discharging, after generation of discharge is confirmed by using a power supply for triggering discharge, a pulse current is applied to the machining gap.